The present invention relates to a composition to enhance the rate of skin cell renewal or exfoliation and to a method of increasing the skin cell renewal rate. In particular, the present invention relates to a composition containing a skin benefit agent that includes a derivative of ascorbic acid that stimulates cell renewal but does not unduly irritate. The present invention also relates to a method of increasing the rate of skin-cell renewal by applying a composition to the skin, wherein the composition comprises an effective amount of a derivative of ascorbic acid.
The skin of humans is continually assaulted by environmental conditions such as the sun, wind, and pollution. These environmental assaults weather or age the skin causing, among other things, wrinkles, age spots and other undesirable skin conditions. In addition, the effects of natural aging also cause the skin to wrinkle.
These negative effects can be prevented or at least ameliorated by applying skin care cosmetics that contain skin benefit agents according to the present invention.
Human skin may be classified into two major parts: the outer layer or epidermis and an underlying layer or dermis. The dermis contains among other things, blood vessels, nerves, collagen, elastin, and fibroblast cells, which are responsible for the biosynthesis of collagen and elastin.
The epidermis itself also may be considered to consist of two major zones, an inner or malpighian layer and an outer or horny layer. The malpighian layer, a living tissue, may be further divided into basal, spinous, and granular layers. The horny layer, a dead tissue, is also referred to as stratum comeum.
In the natural process, basal cells in the basal layer move outward through the spinous and granular layers to become dead cells called corneocytes, in the stratum corneum. The stratum corneum consists of approximately 14 layers of corneocytes. In the normal skin it takes about 14 days for the basal cells to move from the basal layer to the end of the granular layer and to become corneocytes, and another 14 days to reach the outermost layer of the stratum corneum, where they are naturally shed or exfoliated. This process of forming corneocytes is called keratinization, and stratum corneum are the natural products produced by this process. The stratum corneum is the skin tissue that one feels when touching the skin. Usually, it takes about 28 days for cells of the basal layer to move outward to the surface in the course of making new skin.
It is thought that by increasing the natural desquamation rate (the cell renewal rate) of the outermost part of the stratum corneum and thus exposing lower layers of the stratum corneum, the appearance of the skin will be improved. Many substances are known to increase the rate of natural desquamation but recently compositions containing hydroxycarboxylic (alpha and beta) acids have received an increasing amount of attention.
A drawback to the use of hydroxycarboxylic acids is that they are most effective at low pHs, about 4.0 or less. It appears, however, that there exists a strong correlation between the degree of exfoliation (cell renewal rate) and the degree of irritation. Thus, at pH of about 4.0 or less, the hydroxycarboxylic acids show significant stimulation of cell renewal but also a fair degree of skin irritation. On the other hand, as the pH approaches neutral (7.0), the cell renewal rate decreases while there is little or no irritation. Thus, it would be desirable to achieve an increase in the rate of natural desquamation without further increasing the irritation of the skin. It would be most desirable to provide for enhanced skin desquamation at a neutral pH. The present invention solves that problem by providing for enhanced skin desquamation at a neutral pH without undue irritation.